Lake Carlsbad is not a lake in the ordinary sense. It is the Pecos River, backed up behind the upper Tansill Dam into a slow green pool that runs straight through downtown Carlsbad from the railroad bridge down to the dam. The city owns both banks, roughly 125.6 acres of park, and the swimming happens off the west side at the end of Park Drive.

Getting in the water here is sanctioned, not merely tolerated. Carlsbad's municipal code names the Carlsbad Municipal Beach swimming area outright, the city hires seasonal lifeguards for that beach along with the water park next door, and the city's own notices about treating invasive aquatic weeds state that swimming, boating and fishing stay safe during treatment. No admission charge is published for the beach or for river access. The guards are a summer arrangement, so do not count on one in May or September.

What does cost money is the Carlsbad Water Park at 708 Park Drive, a separate chlorinated facility a short walk south of the beach with five slides, a lazy river and a zero entry children's area. Admission is $5, free for anyone under 4 or over 61. It runs Tuesday through Sunday, 11 a.m. to 6 p.m., Memorial Day weekend through Labor Day weekend, and drops to weekends only in August. Slide riders have to be 48 inches. A free splash pad sits at the beach area itself, open 8 a.m. to 8 p.m. daily. The Riverwalk Recreation Center at 400 Riverwalk Drive is free to use but has no pool despite the name, so the year round indoor option in town is the Carlsbad High School Natatorium.

The swim area shares water with boats. A concrete ramp sits just north of it, kayaks and paddle boats rent on the bank, and skiers use the lake. Close to five miles of lighted walkway follow the two banks past tennis and racquetball courts, playgrounds, a skate park, sand volleyball and the Beach Bandshell.

Dogs are excluded twice over. One section of city code keeps them off the Municipal Beach swimming area, and another makes it unlawful to let an animal swim in the Pecos anywhere the river abuts a public park between the flume near Calloway Drive and the Bataan bridge at Greene Street.

Flow here is managed rather than natural. Brantley and Avalon dams sit upstream, the two Tansill dams sit in town, and the river between Lake Carlsbad and Avalon Reservoir is usually dry because that water is diverted into the Carlsbad main canal. Releases can put current under a flat looking surface, so it is worth checking the USGS gauge at Carlsbad before a long swim. Summer air runs past 100 degrees with thin shade on the bank, which catches out more people here than the water does.

On water quality, plainly. New Mexico's environment department carries this reservoir as assessment unit NM-2203.B_00, 134.28 acres, and has not assessed it for primary contact recreation. It is listed as not supporting aquatic life because of DDT and PCB fish consumption advisories, first listed in 2016 and 2010. Those advisories are about eating what you catch, not about getting in. Golden algae, Prymnesium parvum, has caused repeated fish kills in the Carlsbad municipal reservoirs, and it kills gill breathers rather than people. If the water looks oddly dark blue in July, that is the city's aquatic shade dye, applied to starve the invasive weeds of light, and it clears in seven to ten days.